A vital and growing use of energy is in the generation of electricity. In the United States alone, the financial cost of producing electricity is in excess of $60 billion a year; however, the cost of electrical energy should not be measured in dollars alone. When identifying the total cost of electricity, the environmental cost of smog and carbon dioxide pollution associated with electricity production from fossil fuel must be considered. Currently, approximately 20 percent of the electricity produced in the United States is used for lighting.
The most widely used sources of artificial illumination (defined as a source other that the sun), to date, are incandescent and fluorescent lamps. In the near future, however, solid-state lighting (SSL) devices promise to replace conventional light sources. SSL devices: 1) are energy efficient, 2) produce little pollution, 3) are vibration and shock resistant, and 4) are exceptionally long lived. These devices will allow a wide variety of lighting, including artificial lighting that is very similar to natural daylight and, with appropriate circuitry, the color and intensity of the lighting can be controlled. SSL devices also offer more flexible design possibilities. They can be manufactured as flat packages of any shape and can be placed on floors, walls, ceilings, etc. In the United States, through the use of SSLs, the total cost of lighting would be reduced by about $100 billion and pollution would be reduced by over 100 million metric tons between now and the year 2020. By the year 2020, electricity used annually for lighting could be cut by about 50%, sparing the atmosphere 28 million metric tons of carbon emissions annually. As an example of the benefits of SSLs: a typical 12-inch-diameter red traffic signal using a long-life 140 Watt (W) incandescent lamp generates a white flux of nearly 2000 lumens (lm). The red filter that covers the white lamp transmits only about 200 lm of the original intensity. The corresponding LED solution uses LED lamps to produce greater than 200 lm of red light, while consuming a total of only 14 W, including the losses in the power conversion circuit.
SSL sources can be made with either inorganic or organic semiconductors. An example of such an SSL device that uses an inorganic semiconductor material is an LED. The essential elements of this device are an electron-carrying N-layer and a hole-carrying P-layer. When a forward voltage is applied to the structure (negative to the N-layer and positive to the P-layer), electrons are ejected from the N-layer and the holes in the P-layer. Electrons and holes can radiatively recombine, thereby emitting a photon. The wavelength and color of the photon is determined by the difference in the energy levels between the electrons and the holes, which is determined by the properties of the material utilized to fabricate the structure.
Many approaches exist for generating white light using solid state devices, e.g., a multi-chip LED, a blue LED plus yellow phosphor, a blue LED plus green and red phosphors, and a UV-LED plus red, green, and blue phosphors. Conventional inorganic phosphors and group II-VI semiconductor nano-crystals at an excitation wavelength between 370 nanometers (nm) and 460 nm can also be used to generate white light.
Another way to generate SSL white light is to efficiently mix the output of devices providing red, green, and blue light. An SSL device and a phosphor together can also generate white light. For instance, when a blue emitting SSL device shines on a yellow-emitting phosphor, the resulting light appears white to the human eye. However, in this case the color rendering is usually poor. This problem can be overcome by using the light from a UV-emitting device to excite an appropriate color combination of phosphor, to produce white light.
Currently, the LED industry normally uses organic polymeric materials to encapsulate LEDs. These encapsulants include epoxy resins, polycarbonates, siloxane polymers, and dealcoholyzed sol-gels. These encapsulants can also be used as a binder for a Cerium (Ce) and activated Gadolinium (Gd) doped YAG (Yttrium Aluminum Garnet) phosphor, and any other dopant activated to generate white light. This material is typically referred to as Ce:YAG phosphor. The white light emission results from blue LED pumping (typically around 450 nanometers [nm] LED emission) to generate white light appearance. Most of the organic polymers are overly sensitive to UV light and start to degrade after certain periods of exposure. Upon this degradation, yellow to brown color starts to develop in the encapsulant, which has a significant impact on the white light emission because of the absorption of the new species generated in the degraded polymers. The extent of discoloration is dependent upon the UV light wavelength, the UV stability characteristics of the encapsulant material and the UV light exposure time.
Using such unstable materials for LED encapsulant applications has two detrimental consequences. The lifetime of such a device will be undesirably short and, therefore, maintenance costs will be very high. Materials for LED encapsulation should have specific characteristics. The characteristics needed for a good LED encapsulant material are known to those skilled in the art, and include: UV stability, heat stability, and hydrolytic stability of materials designed for such applications. Encapsulant materials for LED applications should tolerate low temperature (about −40° C.) and high temperature (from about 85° C. to 150° C.), and cycling to mimic real life performance in different seasons or different environmental conditions. Cracking can be a major issue in encapsulation of, for example, a phosphor for LED applications because encapsulant cracking can cause wires on an LED to debond.
LED manufacturers currently are using epoxy resins or siloxane materials in such encapsulant applications. The epoxy resins now in use are organic polymeric materials and they have a tendency to yellow upon exposure to UV light. The siloxane materials used are transparent to UV light and can resist the UV light exposure from the LED. However, these siloxane materials are normally very soft and can be damaged very easily during manufacturing, shipping and/or application. In addition, these siloxane materials cannot protect moisture sensitive dopants. A non-hydrolytic sol-gel spin-on glass material can be a good alternative for both of these systems
The prior art reveals there is a need for improving material characteristics for LED encapsulation applications. The major focus of these prior art patents involves using an epoxy or siloxane material as an encapsulant in such LED applications.
U.S. Pat. No. 6,204,523 describes a method for encapsulating a phosphor in an LED using a silicone material. The LED can emit light of wavelength green (570 nm) to near UV (350 nm) wavelength range. This patent claims that the silicon material does not yellow upon extended exposure to UV light. It is also disclosed it is then necessary to use a resilient soft core in the vicinity of the LED chip to prevent damage caused by mechanical stress to the LED chip or its wire leads. A harder shell is also utilized on the exterior to provide better protection and integrity to the LED. Different dopants can be used in these encapsulant materials. The silicone coated LED substantially maintains its light transmission when exposed to a temperature between 85 degrees Celsius (° C.) and 100° C. and a relative humidity of 85%.
The published prior art documents the use of a sol-gel material resulting from the de-alcoholation of an alkoxy silane. The process temperature is between 80° C. and 150° C. The fluorescent material is admixed with a solution of this sol-gel material, which is then applied and heated to produce a glass-like body. Two different dopants were also used with the sol-gel material. These dopants have compositions of SeSEu2+, which emits red fluorescent light, and (Sr, Ba, Ca)S:Eu+2, which emits green fluorescent light. The light-emitting device has a glass-like layer containing said fluorescent materials with a thickness of 100 microns (micrometers) or less.
U.S. Pat. No. 6,351,069B1 describes the use of an epoxy resin paste as a binder (encapsulant) for an LED. A Ce-YAG phosphor is coated onto a gallium nitride (GaN) LED to generate white light from a 470 nm LED. The secondary light from the supplementary fluorescent material allows the device to produce “white” output light that is well balanced with respect to color, for true color rendering applications. The die is preferably a GaN based die that emits light having a peak wavelength of 470 nm. The fluorescent layer also includes a primary fluorescent material in an epoxy resin. Such epoxy materials are sensitive to UV light and suffer from the drawback that their optical transmissive characteristics degrade over time because of yellowing. As a result of such degradation, some of the light produced is absorbed by the encapsulant.
U.S. Patent application number US 2002/0093287A1 describes the addition of an extra diffusion layer onto the yellow fluorescent powder on a blue light LED chip. This diffusion layer, which contains micro-particles such as glass powder or transparent plastics such PMMA, PET, PC and PE, diffuses and refracts the light and makes it more uniform.
U.S. Patent application number US 2002/0084749 A1 describes a multi-layer coating for an LED, which layer contains a silicone encapsulant, a phosphor, a reflecting layer and a layer encapsulant. It is claimed that the reflecting layer reflects the unconverted UV light back into the phosphor layer where it is converted to visible light, resulting in increased output from the light source.
U.S. Pat. Nos. 6,069,440 and 5,998,925 disclose achieving white light conversion by using a phosphor suspended in an epoxy resin as an encapsulant coated on top of the LED. The phosphor consists of yttrium (Y), lutetium (Lu), scandium (Sc), lanthanum (La), gadolinium (Gd), and samarium (Sm) and at least one element selected from the group consisting of aluminum (Al), gallium (Ga), and indium (In).
U.S. patent application number 2001/0030326A1, U.S. Pat. No. 6,245,259B1, U.S. Pat. No. 6,066,861 and U.S. Pat. No. 6,277,301B1 all disclose a method that uses a dopant containing a resin such as an epoxy resin to encapsulate an LED. The semiconductor body emits radiation in the ultraviolet, blue and/or green spectral region and luminescence such as A3B5X12:M (A=Y, Ga, boron [B], Al, Ga, and M=europium [Eu], chromium [Cr]), e.g., YAG:Ce as a conversation element that converts a portion of the radiation into radiation of longer wavelength. This makes it possible to produce an LED that radiate polychromic light, in particular white light.
U.S. patent application number 2002/0084745A1 claims that an LED comprising a light emitting component and a dielectric phosphor powder that absorbs emitted light from the light-emitting source and reemits light of a different wavelength. The dielectric phosphor powder consists of a mixture of phosphor particles and microscopic, spherical dielectric particles with a band gap energy larger than 3 electron volts (eV). Phosphor particles and bubbles or voids can also be used instead of dielectric particles.
U.S. Pat. No. 6,274,924B1 describes an LED package that includes a heat-sinking slug that is inserted into an inner-molded lead frame. This design includes a reflector cup with a thermally conducting sub-mount. Wire bonds extend from the LED to the metal leads. The metal leads are electrically and thermally isolated from the slug. An optical lens may be added by mounting a pre-molded thermoplastic lens and a soft encapsulant or by casting an epoxy resin to cover the LED. The hardness of the epoxy resin is 50-90 Shore D. The encapsulant used was a soft optical silicone material with a 10 Shore A hardness, which fills the area between the LED die and the optical lens and also protects the LED die.
U.S. Pat. No. 5,145,889 relates to a design for an LED chip wherein lead frames are connected to an anode and a cathode of the LED chip. An encapsulant is used to encapsulate both the light emitting device and the lead frames. A buffer layer is formed between the encapsulant and the LED to reduce the stress from the encapsulant onto the LED chip. The encapsulant is an epoxy composition that includes an epoxy resin, a curing agent, a curing accelerator and a triorganothiophosphite.
U.S. Pat. No. 6,407,411B1 claims an LED packaging that includes a thermally conducting, electrically insulating material that enhances the thermal conductivity and structural integrity of the assembly, a UV-resistant encapsulant material, and an integral electrostatic discharge material. The thermally conducting, electrically insulating material creates an electrically insulated, thermally conductive path in the lead frame assembly for the dissipation of power and also acts as a mounting structure, thereby allowing the use of a soft encapsulant material, preferably a silicone material.
EP 1,248,304A2 discloses using a phosphor in different encapsulants such as an epoxide, an acrylic polymer, a polycarbonate, a silicone polymer, an optical glass or a chalcogenide glass, for light conversion. They used 5% to 35% of a dopant in these encapsulants. Recommended were various methods of coating the encapsulant including spraying, screen printing, electrophoresis, and dipping in a slurry solution.
U.S. Pat. No. 5,959,316 describes encapsulating an LED device with a UV curable epoxy resin. The inventor uses a UV curable material because the normal drop in viscosity during the heat cure would allow most resins, especially an epoxy resin, to flow away from the LED header despite the small size of the drop utilized.
Manufacturers are currently using a number of epoxy materials for wafer bonding applications. These epoxy materials are not ultraviolet (UV) light stable and normally degrade during 5 hours exposure to 365 nanometer (nm) UV light at 45-84 milli-watts (mW)/cm2, which results in yellowing and drastic reduction of light transmission. The critical elements of the prior art wafer bonding processes normally involve bonding a clear glass to a fully fabricated wafer using a UV curable epoxy. The silicone side of this glass-epoxy-silicon sandwich is then thinned down to an isolation moat that allows the formation of a P and N back ohmic contact.
The subject invention provides a number of advantages over current materials useful for LED encapsulation or wafer bonding. Such advantages include: (1) improved UV stability of these sol-gel spin-on glass materials over prior art organic polymers; (2) superior thermal and hydrolytic stability over currently used organic materials; (3) improved cracking and yellowing characteristics at −40° C. to 150° C., (4) the capability to host different organic and inorganic dopants for optical and electrical applications; (5) inherently flame retardancy, which means they can be used in various applications, such as in the aerospace industry; (6) a refractive index can be easily tuned by minor changes in the chemistry of the material; (7) the ability to function as a good host for different dopants, especially moisture sensitive and nano crystal materials; (8) good hydrophobicity of these materials enhances their hydrolytic stability; (9) various sol-gel moieties can be introduced into these material systems to lower oxygen and moisture permeability and block any UV irradiation from an LED, for safety purposes; (10) depending on the application, a wide range of different film thicknesses (e.g., 1-100 microns) can be applied; (11) these materials can be UV curable, making them useful for printing different structures on top of electronic and/or optoelectronic wafers.